The present invention relates to an electrophotographic image forming apparatus, such as a copying machine, a printer, a facsimile, or a multifunction apparatus formed by combining them, and in particular, to an improvement in a cleaning technique for an image forming apparatus.
Up to now, electrophotographic (electrostatic transfer type) image forming apparatuses, such as copying machines and printers, have been widely known. In such image forming apparatuses, a toner image is transferred to a recording sheet by a final transfer member, and then the toner image is fixed to thereby realize on the recording sheet the toner image as a permanent image. The portion of the toner that is not transferred to the recording sheet has to be removed by a cleaning device in the image forming apparatus. As a method for removing such residual toner, there have been proposed a technique in which a blade is used (See, for example, JP6-148910A), a technique in which a brush is used (See, for example, JP 10-274889 A), and so on.
However, these conventional techniques have the following problem.
When a blade is used, cleaning is performed with the blade being held in press contact with an intermediate transfer member, so that wear of the intermediate transfer member progresses after use over a relatively short period of time, so that the service life of the intermediate transfer member is rather short. Further, a surface flaw of the intermediate transfer member generated through wear may cause disturbance in the toner image. In particular, when a spherical toner produced by polymerization is used as the toner for forming the toner image, it is difficult to maintain a satisfactory cleaning performance.
When a brush is used, in order to remove residual toner from the brush, it is necessary for the cleaning device to be provided with, apart from the brush, a detoner roll (a roll for transferring residual toner), a flicker member (a member for shaking off residual toner) or the like, which is rather disadvantageous in terms of space efficiency and cost. Instead of using a detoner roll or a flicker member, it might be possible to adopt an arrangement in which residual toner is temporarily retained by the brush and in which apart from the image formation mode, there is provided a special cleaning mode in which the temporarily retained toner is carried to a residual toner recovery portion by utilizing a potential gradient. However, that would involve frequent operation of the cleaning mode, thus impairing the productivity in continuous image formation.
The present invention has been made in view of the above-mentioned technical problem, and provides an image forming apparatus in which an increase in the service life of the intermediate transfer member is compatible with a reduction in apparatus size and cost and which provides high productivity in image formation.
Therefore, according to an aspect of the present invention, an image forming apparatus includes: an image bearing member; an image forming unit which forms a toner image on the surface of the image bearing member; an intermediate transfer device formed by at least one intermediate transfer member and which is in contact with the image bearing member at a primary transfer position; a final transfer member which is in contact with the intermediate transfer device at a final transfer position; a potential gradient forming unit which forms a potential gradient between the image bearing member, the intermediate transfer member, and the final transfer member so as to transfer the toner image to the final transfer position; and a cleaning unit which removes residual toner on the intermediate transfer member that is not finally transferred to a recording sheet at the final transfer position. In the apparatus, the cleaning unit includes: an upstream side cleaning roll which is in contact with the intermediate transfer member, a downstream side cleaning roll which is in contact with the intermediate transfer member on the downstream side of the upstream side cleaning roll with respect to a feedback direction in which the residual toner is fed back from the final transfer position to the primary transfer position, and a cleaning brush which is in contact with the intermediate transfer member on the downstream side of the upstream side cleaning roll with respect to the feedback direction and on the upstream side of the downstream side cleaning roll with respect to the feedback direction.
In this way, the blade is not held in press contact with the intermediate transfer member, so that wear of the intermediate transfer member does not progress easily, thus making the service life of the intermediate transfer member relatively long. Further, there is no need to provide a detoner roll (a roll for transferring residual toner), a flicker member (a member for shaking off residual toner) or the like for the cleaning brush, which is advantageous in terms of space efficiency and cost. Further, since this cleaning brush is placed between an upstream side cleaning roll and a downstream side cleaning roll, it is possible to remove residual toner without providing any special cleaning mode, and there is no fear of the productivity in image formation being impaired.
Here, the image forming apparatus according to the present invention may be structured such that the upstream side cleaning roll and the downstream side cleaning roll are all formed of conductive members, and that the potential gradient forming unit applies to one cleaning roll a high cleaning roll bias voltage higher than the surface potential of the intermediate transfer member which is in contact with the cleaning roll, and applies to the other cleaning roll a low cleaning roll bias voltage lower than the surface potential of the intermediate transfer member which is in contact with the cleaning roll.
Further, the image forming apparatus according to the present invention maybe structured such that the potential gradient forming unit applies to the cleaning brush a cleaning brush bias voltage which is on the side closer to the side of the cleaning roll bias voltage to be applied to the upstream side cleaning roll as compared with the surface potential of the intermediate transfer member which is in contact with the cleaning brush.
Further, the image forming apparatus according to the present invention may be structured such that the cleaning unit is equipped with a cleaning blade which scrapes off residual toner adhering to the (upstream side and/or downstream side) cleaning roll(s) and a take-out mechanism which takes out the residual toner scraped off by the cleaning blade. Furthermore, the cleaning roll and the cleaning blade may be formed of metal.
Further, the present invention is applicable not only to a monochrome image forming apparatus, but also to a multicolor image forming apparatus. That is, it is possible to adopt a construction which has as the image bearing member plural image bearing members for different colors and has as the intermediate transfer device a single intermediate transfer member. It is also possible to adopt a construction which has as the image bearing member plural image bearing members for different colors and has as the intermediate transfer device a first upstream side intermediate transfer member which is in contact with a part of the plural image bearing members, a first downstream side intermediate transfer member which is in contact with the remaining ones of the plural image bearing members, and a second intermediate transfer member which is in contact with the first upstream side intermediate transfer member and the first downstream side intermediate transfer member and to which a toner image is transferred from the first downstream side intermediate transfer member after the transfer of a toner image from the first upstream side intermediate transfer member, the final transfer member being in contact with the second intermediate transfer member.
As a specific construction of the latter image forming apparatus, it is possible to adopt a construction such that the image forming apparatus includes: as the image bearing member four image bearing members for yellow, magenta, cyan, and black; and as the intermediate transfer device a first upstream side intermediate transfer member and a first downstream side intermediate transfer member respectively in contact with two of the four image bearing members, and a second intermediate transfer member which is in contact with the first upstream side intermediate transfer member and the first downstream side intermediate transfer member and to which a toner image is transferred from the first downstream side intermediate transfer member after the transfer of a toner image from the first upstream side intermediate transfer member, in which the final transfer member is in contact with the second intermediate transfer member.
In this image forming apparatus, the upstream side cleaning roll abuts against the second intermediate transfer member, and the downstream side cleaning roll may be exclusively in contact with the first upstream side intermediate transfer member or exclusively in contact with the first downstream side intermediate transfer member; there may be provided as the downstream side cleaning roll two downstream side cleaning rolls, one of which is in contact with the first upstream side intermediate transfer member and the other of which is in contact with the first downstream side intermediate transfer member.
Further, this image forming apparatus may have as the cleaning brush only an upstream side cleaning brush which is in contact with the second intermediate transfer member and a downstream side cleaning brush which is in contact with the first upstream side intermediate transfer member, or only a downstream side cleaning brush which is in contact with the first downstream side intermediate transfer member; or, it may have as the downstream side cleaning brush two cleaning brushes, one of which is in contact with the first upstream side intermediate transfer member and the other of which is in contact with the first downstream side intermediate transfer member.
Further, of the intermediate transfer member, the cleaning roll abutting against the intermediate transfer member, and the cleaning brush abutting against the intermediate transfer member, at least two may receive application of bias voltage from the same power source. Specifically, it is possible to adopt construction in which grounding is possible from the same power source to the intermediate transfer member, the cleaning roll abutting against the intermediate transfer member, and the cleaning brush abutting against the intermediate transfer member through an electrical resistor or a zener diode, or a construction in which bias voltage is supplied from the power source through an electrical resistor or a diode.
Further, the potential gradient forming unit can apply biases of the same polarity (positive or negative) to the intermediate transfer member, the cleaning roll abutting against the intermediate transfer member, and the cleaning brush abutting against the intermediate transfer member. It is possible to adopt a construction in which the cleaning member (the cleaning roll or the cleaning brush) does not come into contact with the final transfer member, or a construction in which the cleaning member (the cleaning roll or the cleaning brush) does not come into contact with the image bearing member. By thus constructing the image forming apparatus, it is only necessary to prepare a positive or a negative power source, which contributes to a reduction in apparatus size and is advantageous in terms of cost.
In accordance with the present invention, it is possible to provide an image forming apparatus in which an increase in the service life of the intermediate transfer member and a reduction in apparatus size and cost are mutually compatible and which involves as little deterioration in productivity in image formation as possible.